Optical film and process for producing the same

ABSTRACT

An optical film composed of a thermoplastic resin film obtained by using a melt extruding machine, characterized in that said thermoplastic resin film satisfies a relation of the formula below over the whole surface of the film when an angle made by the extruding direction of the thermoplastic resin film from the melt extruding machine and a slow phase axis at each point is α, and a retardation amount at each point is Re, is provided. [sin 2  2α]×[sin 2 (π·Re/550)]≦4.0×10 −5 . According to the present invention, an optical film wherein there is no problem on a residual solvent, optical distortion is small, and there is no problem of color unevenness and color absence of a liquid crystal display obtained by using the optical film as a protective film of a polarizer is provided.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an optical film suitably used as avariety of films used in a flat panel display, etc. and a raw materialfilm thereof, a production method of the optical film, a protective filmof a polarizer composed of the optical film, a polarizing film havingthe protective film and a phase difference film obtained by performingstretch processing on the optical film.

BACKGROUND ART

In a liquid crystal display and other flat panel displays, a phaseplate, a polarizing plate and a liquid crystal cell substrate, etc.configured by including a thermoplastic resin film are used. As thephase plate, those obtained by stretching a raw material film, such aspolycarbonate, giving a certain amount of retardation (also referred toas a phase difference) thereto, and putting two or more of them togetherare mainly used.

As the polarizing plate, those obtained by stacking a protective film onan upper surface and a lower surface of a polarizer made by polyvinylalcohol are mainly used. As the protective film of a polarizer, a filmobtained by a solution casting method of triacetyl cellulose is mainlyused. As the liquid crystal substrate film, a film made by polyethyleneterephthalate (PET) is mainly used.

Along with a recent flat panel display becoming larger, thinner andfiner and to have an increased contrast, the above various thermoplasticresin films have been required to have highly less optical distortionthan those in the conventional ones. It is because when the opticaldistortion is large, optical defects, such as color unevenness and colorabsence, etc., allover the flat panel display are caused.

Conventionally, as an index for indicating optical distortion of variousthermoplastic resin films, an amount of retardation has been mainlyused. By making the value of the retardation amount small or apredetermined value and uniform on a plane, an effort to diminish theoptical distortion has been made. For example, in a polarizer protectivefilm, having a small and constant retardation amount allover the filmhas been its index.

An optical film made by a thermoplastic resin wherein a retardationamount is small and constant allover the film has been proposed.

As a polarizer protective film, conventionally, a film made by asolution casting method of triacetyl cellulose (TAC) has been mainlyused. A solution casting method film of TAC has a relatively smallretardation being relatively constant on a film surface. However, thereare disadvantages that it is poor in productivity, a solvent cannot becompletely removed at the time of drying after the solution casting, andan emitted solvent gives an adverse effect on an electronic circuit andother parts of the flat panel display to cause an erroneous operationand display defective, etc.

Therefore, recently, as a protective film, a film obtained by a meltextrusion method of a thermoplastic resin has been studied as substitutefor the film by a solution casting method of TAC. For example, theJapanese Unexamined Patent Publication No. 2000-273204 discloses atechnique of obtaining a sheet having a sheet thickness of 150 to 1000μm, in-plane thickness tolerance (Rmax) on the sheet of 15 μm, roughnessof the sheet surface of 0.06 μm, and a retardation amount (maximumvalue) on the sheet surface of 15 nm, wherein a retardation amount isrelatively constant and relatively small by performing melt extrusionmolding on a specific thermoplastic resin under a specific condition.

However, the technique disclosed in the publication is for a thick sheetof 150 μm or more, generally, the thinner the sheet thickness becomes,the larger the thickness unevenness tends to be comparing with thethickness, and optical distortion becomes worse. Therefore, there hasbeen a problem that it was impossible to respond to a recent demand fora thinner flat display.

Also, in the technique disclosed in the publication, there was a problemthat a maximum value of an in-plane retardation amount was as much as 15nm, which caused large optical distortion, so that it was impossible torespond to required performance of a flat panel display with stillhigher performance.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an optical film havingno residual solvent problem, small optical distortion, and no problem ofcolor unevenness and color absence of a liquid display to be obtainedwhen used as a polarizer protective film, a process for producing theoptical film, a polarizer protective film configured by the opticalfilm, a polarizing film having the protective film, and a phasedifference film with small optical distortion obtained by performingstretch processing on the optical film.

The present inventors have been committed themselves to study forsolving the above problems, consequently, found a technique of making aretardation amount (Re) of an optical film to be obtained to be 10 nm orless by optimizing a melt extrusion condition and an operation conditionof cooling drums, and completed as another invention.

When the maximum value of Re becomes 0 nm, it means there is no in-planeoptical distortion. However, even by using such a technique, the maximumvalue of Re is hardly made to be a certain level or less and there is alimit. Thus, the present inventors focused not only on the Re value buton the direction of a slow phase axis indicating directivity. At eachpoint, the direction that birefringent becomes maximum is referred to asa slow phase axis, and the direction that the birefringent becomesminimum is referred to as a fast phase axis.

The present inventors found that it is more important to control Re in arelation of an angle α made by the slow phase axis and the extrudingdirection of a thermoplastic resin than to only focus on Re to diminishit; a Z value expressed by “Z=[sin² 2α]×[sin²(π·Re/550)]” is suitable asthe index; while it is preferable that the Z value is small, colorunevenness and color absence of a liquid crystal display to be obtainedlargely change on a boundary of 6.0×10⁻³, and when being at this valueor smaller, it is possible to respond to the recent high demands for aliquid crystal display. The present invention was completed based on theknowledge.

Namely, according to the present invention, there is provided

-   -   an optical film composed of a thermoplastic resin film obtained        by using a melt extruding machine, characterized in that    -   said thermoplastic resin film satisfies a relation of the        formula below over the whole surface of the film when an angle        made by the extruding direction of the thermoplastic resin film        from the melt extruding machine and a slow phase axis at each        point is α, and a retardation amount at each point is Re.        [sin² 2α]×[sin²(π·Re/550)]≦4.0×10⁻⁵

Preferably, a value of said Re is 10 nm or less.

Preferably, said thermoplastic resin is an alicyclic structurecontaining polymer.

The optical film according to the present invention is preferablyproduced, for example, by a production method below.

Namely, according to an example of the present invention, there isprovided

-   -   a production method of an optical film composed of a        thermoplastic resin film, including a step of cooling a molten        thermoplastic resin extruded from an extruding machine by making        it successively circumscribed with a first cooling drum, a        second cooling drum and a third cooling drum:        -   wherein, when assuming that rotation speed of said third            cooling drum is R₃ (m/min.), and rotation speed of said            second cooling drum is R₂ (m/min.), a ratio of the R₃ and R₂            (R₃/R₂) is made to be 0.990 or more but less than 0.999 to            cool said thermoplastic resin.

In the production method, it is preferable that, when assuming a resincontact time in said first cooling drum is t₁ (sec.), a temperature whensaid thermoplastic resin moves away from said first cooling drum is Tp₁(° C.), and a glass transition temperature of said thermoplastic resinis Tg (° C.), t₁×(Tp₁−Tg) (unit: sec.·deg) is made to be −50 or higherand 20 or lower to cool said thermoplastic resin.

In the production method, it is preferable that, when assuming thatrotation speed of said first cooling drum is R₁ (m/min.), a ratio of theR₂ and R₁ (R₂/R₁) is made to be 0.990 or more but less than 1.01 to coolsaid thermoplastic resin.

In the production method, it is preferable that, when assuming that atemperature when said thermoplastic resin moves away from said thirdcooling drum is Tp₃ (° C.) the Tp₃ is made to be a lower temperaturethan said Tg by 50 to 100° C. to cool said thermoplastic resin.

In the production method, it is preferable that, when assuming that atemperature when said thermoplastic resin moves away from said secondcooling drum is Tp₂ (° C.), the Tp₂ is made to be a lower temperaturethan said Tg by 0 to 60° C. to cool said thermoplastic resin.

In the production method, it is preferable that, a temperaturedifference of said first cooling drum and said second cooling drum ismade to be 20° C. or less to cool said thermoplastic resin.

According to the present invention, there is provided a protective filmof a polarizer composed of any one of the above optical films.

According to the present invention, there is provided a polarizing filmhaving a polarizer and the above protective film stacked on one surfaceor both surfaces of the polarizer via an adhesive layer.

According to the present invention, there is provided a phase differencefilm obtained by performing stretch processing on any one of the aboveoptical films.

BEST MODE FOR CARRYING OUT THE INVENTION

An optical film according to the present invention is configured by athermoplastic resin film.

The thermoplastic resin film used in the present invention includes atleast a thermoplastic resin.

The thermoplastic resin is not particularly limited as far as it is aresin normally used in producing an optical film. For example,polyethylene, polypropylene, an ethylene-propylene copolymer,polystyrene, polyacrylonitrile, an acrylonitrile-styrene copolymer,polyvinylchloride, polymethyl methacrylate, polyethylene terephthalate,polybutylene terephthalate, polycarbonate and an alicyclic structurecontaining polymer, etc. may be mentioned. Among them, because of hightransparency and excellent film strength, polyethylene terephthalate,polybutylene terephthalate and an alicyclic structure containing polymerare preferable and, furthermore, an alicyclic structure containingpolymer is particularly preferable because a phase difference is easilymade small.

An alicyclic structure containing polymer is a polymer having a cyclicstructure made by carbon-carbon saturated bonds (referred to as “analicyclic structure” in the present invention) in a repeat unit and, forexample, a well-known polymer disclosed in the Japanese UnexaminedPatent Publication No. 2002-321302 may be used. For example, ring-openedpolymers and the hydrogenated products of monomers having a norbornenering (hereinafter, referred to as “norbornene”), addition polymers ofnorbornene and the hydrogenated products, addition copolymers ofnorbornene and a vinyl compound and the hydrogenated products; polymersobtained by hydrogenating aromatic ring of a polymer of polystyrene orother aromatic vinyl hydrocarbon compounds, addition polymers ofmonomers having an alicyclic structure and a vinyl group, additionpolymers of monomers having one or more unsaturated bonds in acarbon-carbon cyclic structure and the hydrogenated products, etc. maybe mentioned.

The thermoplastic resin film used in the present invention may beblended with a variety of compounding agents in accordance with needother than the above thermoplastic resins. The compounding agents arenot particularly limited. For example, antioxidants, heat stabilizers,light stabilizers, weather resistant stabilizers, ultraviolet rayabsorbents, near infrared ray absorbents and other stabilizers;lubricants, plasticizers and other resin improvers; colorants, pigmentsand other coloring agents; antistatic agents, etc. may be mentioned.These compounding agents may be used alone or in combination of two ormore kinds. A blending quantity of these compounding agents is suitablyselected in a range of not hindering the objects of the presentinvention.

As antioxidants, phenol based antioxidants, phosphorus basedantioxidants, and sulfur based antioxidants, etc. may be mentioned.Among these, phenol based antioxidants are preferable andalkyl-substituted phenol based antioxidants are particularly preferable.By including an antioxidant, coloring and strength decline due todeterioration by oxidization, etc. at the time of molding a moldedproduct (a thermoplastic resin film) can be prevented withoutdeteriorating transparency and a low water absorbing property of athermoplastic resin film after melt extrusion molding, which will beexplained later on. The antioxidants may be used alone or in combinationof two or more kinds. A blending quantity of antioxidants is suitablyselected in a range of not hindering the object of the presentinvention. Normally, it is 0.001 to 5 parts by weight or so andpreferably 0.01 to 1 part by weight with respect to 100 parts by weightof a thermoplastic resin.

The thermoplastic resin film used in the present invention is producedby performing melt extrusion molding on a molding material including atleast a thermoplastic resin. Specifically, first, a molding material isobtained by, for example, mixing a thermoplastic resin and a variety ofcompounding agents. Next, the molding material is processed to bematerial pellets suitable for melt extrusion molding. Next, the materialpellets are supplied to a melt extruding machine and processed to be afilm shape by a melt extrusion method to obtain a thermoplastic resinfilm. The melt extrusion method is a method of heating and melting thematerial pellets in a cylinder, applying pressure by a screw, andextruding from a die, such as T-die, to be, for example, a film shape.Normally, a thermoplastic resin in a molten state extruded from a die issuccessively transferred by being circumscribed with a plurality ofcooling drums, cooled during that time, then, subjected to necessarysteps to be a thermoplastic resin film. A width of the thermoplasticresin film after passing through the cooling drums becomes narrower by 2to 10% due to neck-in comparing with a width of a molten thermoplasticresin immediately after being extruded from a die (same as a width of alip of the die). An end of the thermoplastic resin film in the widthdirection (hereinafter., also simply referred to as “an end”) has athicker thickness and larger optical distortion comparing with those onother parts of the film.

In the present invention, since the thermoplastic resin film aftercooling is for an optical use, normally, sides thereof are suitably cutoff before winding with a wind drum. The cut off parts will be called“selvage” in the present invention. A part of the cooled thermoplasticresin film yet to be stretched after removing parts to be the selvagewill be referred to as “an optical film” in the present invention. As ina protective film and a liquid crystal substrate, etc., when used in astate that a retardation amount is nearly zero, selvage is removed fromthe thermoplastic resin film yet to be stretched, the optical film istaken out and wound by a wind drum.

The thermoplastic resin film used in the present invention is made to bean optical film, wound to be a roll shape by a wind drum, then, it maybe subjected to the next step: a stretch step and an adhesion step toother film, etc. to be value-added.

An average thickness of the thermoplastic resin film used in the presentinvention is not particularly limited. To make an optical film taken outfrom the thermoplastic resin film used in the present invention suitableto a thin flat panel display, etc., it is preferable that an averagethickness of the thermoplastic resin film is made to be normally 100 μmor less, preferably 80 μm or less, particularly preferably 60 μm orless. An average thickness of the thermoplastic resin film can be freelyset by changing a feeding speed of the material pellets fed to the meltextrusion machine or a rotation speed of the cooling drum or the both,etc.

The thermoplastic resin film used in the present invention satisfies therelation of the formula below on the whole surface of the film whenassuming that an angle made by the extruding direction from the meltextrusion machine and a slow phase axis at each point is α, and aretardation amount at each point is Re.[sin² 2α]×[sin²(π·Re/550)]≦4.0×10⁻⁵

In the present invention, the left side of the equation [sin²2α]×[sin²(π·Re/550)] is referred to as a Z value. The Z value is anamount corresponding to light leakage caused by retardation of theoptical film. Linear polarization cannot transmit a polarizer having anabsorption axis perpendicular to the light axis. However, when thelinear polarization transmits the optical film with optical distortion(retardation is not zero), a phase delays in the slow phase axisdirection of the optical film, and a part of the linear polarization cantransmit the polarizer. When a part of linear polarization transmits thepolarizer, color unevenness and color absence are caused in a liquidcrystal display using the optical film.

In the above formula, as a wavelength of linear polarization, 550 nmwhich is a center wavelength of visible light is applied, and α isnormally defined as an angle made by a slow phase axis of the film and alight axis of linear polarization. However, when conducting a test ofperformance of an optical film alone, α cannot be defined. Therefore,the present inventors focused on an angle made by the extrudingdirection of the thermoplastic resin film to be an optical film from themelt extruding machine and a slow phase axis at each point, and used thesame as a to define a Z value for an alternative characteristic.

In recent years, as liquid crystal displays become larger, long opticalfilms in roll shape, etc. are put together or such an optical film isput together with other long film to obtain a stacked body in manycases. Thus, defining α in relation with the longitudinal direction(same as the extruding direction from the melt extruding machine) of thestacked body is considered advantageous for managing quality of theoptical film alone.

Values of Re and α at each point as explained above can be measured byusing a phase difference meter, etc., and a Z-value can be obtained fromthe values.

A maximum value of the Z value can be obtained by measuring over asuitable longitudinal length and measuring points including a part closeto an end of the optical film in the width direction. The value can beapplied as a substitute value. The maximum value of the Z value isnormally required to be 4.0×10⁻⁵, preferably 3.8×10⁻⁵, and furthermorepreferably 3.4×10⁻⁵. When the Z value is a predetermined value or less,a problem of color unevenness and color absence of a liquid crystaldisplay to be obtained becomes furthermore smaller, and color unevennessand color absence are not visually sensed, so that the display qualityis remarkably improved, which is preferable.

In the present invention, the maximum value of Re is not particularlylimited. However, Re over whole surface of the film is preferably 10 nmor less, more preferably 7 nm or less, and furthermore preferably 5 nmor less. When the Re value is a predetermined value or less, a problemof color unevenness and color absence of a liquid crystal display to beobtained becomes furthermore smaller. Also, color unevenness and colorabsence after a durability test become furthermore smaller, which ispreferable.

The Re value can be measured by an optical method, such as anellipsometer. As the maximum value of Re, a value obtained by measuringover a suitable longitudinal length and measuring points including apart close to an end of the optical film in the width direction can beapplied as a substitute value.

A method of producing an optical film according to the present inventionwhich satisfies the above conditions is not particularly limited. Forexample, a method of increasing a ratio of the selvage part to the wholefilm so as to satisfy the above conditions on the whole surface of thefilm may be mentioned. The ratio of the selvage part in this case isnormally 3% or more on the right and the left, preferably 5% or more onthe right and the left, particularly preferably 7% or more on the rightand the left, preferably 40% or less on the right and the left, andparticularly preferably 20% or less on the right and the left. Otherthan that, an optical film of the present invention can be also producedby optimizing the conditions from melt extrusion to winding of thethermoplastic resin film.

Below, an example of a preferable production method of the optical filmaccording to the present invention will be explained.

A production method of an optical film according to the presentinvention includes a step of cooling a molten thermoplastic resinextruded from an extruding machine by making it circumscribed with afirst cooling drum, second cooling drum and third cooling drumsuccessively. After that, more preferably, a step of cutting both endportions in the width direction of the cooled thermoplastic resin, and astep of winding the cut thermoplastic resin in a roll shape areincluded.

Here, in the cooling step, the thermoplastic resin is cooled to satisfypreferably at least any one of (1) to (6) below, more preferably atleast (1) and (2) below, and particularly preferably all of (1) to (6)below.

(1) When assuming that rotation speed of the third cooling drum is R₃(m/min.) and rotation speed of the second cooling drum is R₂ (m/min.),the ratio of the R₃ and R₂ (R₃/R₂) is made to be preferably 0.990 ormore but less than 0.999, and more preferably 0.995 or more but lessthan 0.998 to cool the thermoplastic resin. When the value of R₃/R₂ isexcessively large, a molten thermoplastic resin extruded from theextruding machine (hereinafter, also referred to as “a sheetthermoplastic resin”) is stretched and a retardation amount and theunevenness become large, which is not preferable. On the other hand,when the value of R₃/R₂ is excessively small, the sheet thermoplasticresin becomes slack and droops, the weight becomes tension to stretchthe sheet thermoplastic resin, and a retardation amount and theunevenness become large, which is also unfavorable. To determine a setvalue of R₃/R₂, the resin temperature is set to be appropriate for acontraction percentage of the resin caused by a temperature decline fromaround the second cooling drum temperature to around the third coolingdrum temperature when transferring the sheet thermoplastic resin fromthe second cooling drum to the third cooling drum. When the aboverotation speed ratio is applied, it is possible to produce athermoplastic resin film with a small and uniform retardation as aresult that the sheet thermoplastic resin does not become slack and isstretched with a suitable tension.

(2) When assuming that a resin contact time in the first cooling drum ist₁ (sec.), a temperature when the thermoplastic resin moves away fromthe first cooling drum is Tp₁ (° C.), and a glass transition temperatureof the thermoplastic resin is Tg (° C.), t₁×(Tp₁−Tg) (unit: sec.·deg) ismade to be preferably −50 or higher and 20 or lower, and more preferably−40 or higher and +15 or lower to cool the thermoplastic resin. When thevalue of t₁×(Tp₁−Tg) is in the ranges, uniformity of a thickness of anoptical film to be obtained Dr/D_(ave) becomes small, and the maximumvalue of Re becomes small, so that the optical film of the presentinvention can be easily obtained.

(3) When assuming that rotation speed of the first cooling drum is R₁(m/min.), the ratio of R₂ and R₁ (R₂/R₂) is preferably set to 0.990 ormore but less than 1.01, and more preferably 0.995 or more but less than1.000 to cool the thermoplastic resin. When the value of R₂/R₁ is in theranges, unevenness of a retardation amount of an optical film to beobtained becomes particularly small, and wrinkles when winding is hardto arise.

(4) When assuming that a temperature when the thermoplastic resin movesaway from the third cooling drum is Tp₃ (° C.), the Tp₃ is set to belower than the above Tg preferably by 50 to 100° C., and more preferablyby 60 to 80° C. to cool the thermoplastic resin. When the value of theTp₃ is in the ranges, unevenness of a retardation amount of an opticalfilm to be obtained becomes particularly small, and wrinkles whenwinding is hard to arise. To make the temperature Tp₃ in the aboveranges, it is sufficient to control the temperature of the third coolingdrum and the second cooling drum.

(5) When assuming that the temperature when the thermoplastic resinmoves away from the second cooling drum is Tp₂ (° C.), the Tp₂ is set tobe lower than the above Tg preferably by 0 to 60° C., and morepreferably by 20 to 40° C. to cool the thermoplastic resin. When thevalue of the Tp₂ is in the ranges, unevenness of a retardation amount ofan optical film to be obtained becomes particularly small, and wrinkleswhen winding is hard to arise.

(6) A temperature difference of the first cooling drum and the secondcooling drum is made to be preferably 20° C. or less, and morepreferably 10° C. or less to cool the thermoplastic resin. By cooling bymaintaining the temperature difference of the two to be 20° C. or less,the maximum value of retardation can be made small and displayunevenness can be reduced.

The optical film according to the present invention is suitable as aprotective film of a polarizer of a liquid crystal display. Thepolarizer is obtained by performing stretch processing after doping withiodine on a film made by a vinyl alcohol based polymer, such aspolyvinyl alcohol and partially formalization polyvinyl alcohol.

To protect the polarizer, the optical film of the present invention isstaked as a protective film on one surface or both surfaces of thepolarizer via a suitable adhesive layer, so that a polarizing film canbe obtained. As the adhesive layer, an adhesive agent, etc. wherein asuitable polymer is a base polymer, such as an acryl based polymer, asilicon based polymer, polyester, polyurethane, polyether, and syntheticrubber is used.

The optical film of the present invention is suitable to a material forproducing a polarizing film as a protective film by putting thelongitudinal direction together with the longitudinal direction of along polarizer (referred to as roll-to-roll), and adhering to stack.

The optical film of the present invention may be furthermore subjectedto stretch processing to obtain a phase difference film. A phasedifference film is a film having a uniform retardation amount of apredetermined value over the whole surface of the phase difference film.There are phase difference films of λ/2 and λ/4, etc. of a lightwavelength λ to be used. For example, a tenter is used for performingstretch processing on the optical film of the present invention. A filmtemperature during stretching is, for example, (Tg−100)° C. or higherand (Tg +40)° C. or lower. Here, Tg is a glass transition temperature ofa thermoplastic resin to be a material. Also, while the stretching ratiodepends on a retardation value to be obtained and a thickness of thephase difference film, it is normally 1.05 times or more and 3.0 timesor less in the length direction (extruding direction of thethermoplastic resin film) and 0.2 times or less in the width direction,or may be uniaxially stretched in the length direction.

EXAMPLES

Below, the present invention will be explained more in detail by takingexamples and comparative examples. “Part” and “%” in the examples arebased on weight unless otherwise mentioned.

Preparation of various samples and tests were performed as below.

(1) Average Thickness D_(ave) of Film:

By using a contact type film thickness meter, film thickness wasobtained by measuring at every 500 mm over 10 m in the film lengthdirection (20 points) and at 5 points at regular intervals in the widthdirection (unit: μm).

(2) α Re and Z Value and Maximum Values of Re and Z Value:

By using a phase difference measuring device (KOBRA-21ADH made by OjiScientific Instruments), α and Re (unit:nm) were measured at every 500mm over 10 m in the film length direction (20 points) and at 5 points atregular intervals in the width direction, a Z value was calculated fromthe results, and the maximum values of Re and Z value were obtained fromthe values. A measurement wavelength was set to be 550 nm and anincident angle was 0°.

(3) Whole Light Transmittance:

As a simple test of color unevenness when using an optical film obtainedin the present experiment in a flat panel display, the whole lighttransmittance of the optical film was measured by the cross Nicolemethod based on JIS-K7105, and the average value was obtained (unit: %).The value is ideally 0, but preferable when 0.03% or less and defectivewhen 0.04% or more.

(4) Color Unevenness:

Color unevenness and color absence which become a problem when used as aprotective film of a polarizer were tested simply. The configuration ofa liquid crystal display on market normally is that both surfaces of thepolarizer are sandwiched by two protective films to obtain a polarizingplate, furthermore, both surfaces of the liquid crystal are (via othernecessary layers) sandwiched by two polarizing plates. When there isoptical distortion on the protective film, color unevenness and colorabsence allover the liquid crystal display are caused. Opticaldistortion of the optical film to be each protective film was testedsimply as below. By preparing two polarizers on market of liquid crystalpolyvinyl alcohol doped with iodine, the two polarlizers were puttogether so that their polarizing axes were at right angles to eachother, an optical film obtained in the experiment was sandwiched betweenthem to obtain three-layer structure made by the polarizer/opticalfilm/polarizer. Then, transmittance degree of an unpolarized light(natural light) was visually observed. In the present invention, anangle made with the length direction (the extruding direction of thethermoplastic resin film from a melt extruding machine) was defined as αand used as an alternative characteristic. Therefore, to confirm inother directions, leakage of the light was observed in respectivedirections on the film at respective measuring points by rotating thepolarizer. Those with no light leakage observed were marked {circle over(∘)}, those hardly observed were ∘, those observed little were Δ, thoseclearly observed were x. The measurement points in the present inventionwere at every 500 mm over 10 m in the film length direction and at 10points at regular intervals in the width direction.

(5) Simple Liquid Crystal Display Test:

Color unevenness and color absence which become a problem when used as aprotective film of a polarizer were tested simply by using an actualliquid crystal display. A polarizing film was peeled from a liquidcrystal cell very carefully on a display portion of a cellular phone onmarket (black and white two-graduation display, reflection type, adisplay area of 30 mm×30 mm), and a polarizing film produced by usingthe optical film of the present invention was stacked as a protectivefilm on both surfaces of the polarizer to produce a test liquid crystaldisplay. A part close to the end of the optical film in the widthdirection was used as a sample piece, and 10 mass % of polyvinyl alcoholsolution was used for stacking. The above test liquid crystal displaywas produced for each experiment example, a variety of characters andstill images were displayed, and observed from viewable angles of 0° and30° to visually observe existence of color unevenness and color absence(a part where a proper color is not displayed). Those at an approvablelevel were marked ∘, defectives were marked x, and those in the middleand not at an approvable level were marked Δ.

(6) Color Unevenness After Durability Test:

A test piece of 1000 mm×1000 mm was cut from the obtained optical film,and a high temperature high humidity durability promotion test at 80° C.and 90% RH for 100 hours was conducted thereon. After that, the samecolor unevenness test as that in (4) was conducted on the taken out testpiece.

Example 1

In a nitride atmosphere, 500 parts of dehydrated cyclohexane was mixedwith 0.82 part of 1-hexene, 0.15 part of dibutylether and 0.3 part oftriisobutyl aluminum in a reactor at the room temperature, then, whilebeing kept at 45° C., a monomer mixture composed of 70 parts oftricyclo[4.3.0.1^(2,5)]deca-3,7 diene (dicyclopentadiene, DCP), 70 partsof tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodeca-3-ene (tetracyclo dodecene,TCD), and 60 parts of8-ethylidene-tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodeca-3-ene (ethylidenetetracyclo dodecene, ETD); and 40 parts of tungsten hexachloride (0.7%toluene solution) were added continuously for two hours to bringpolymerization. After that, the polymerized solution was added with 1.06parts of butylglycidyl ether and 0.52 part of isopropyl alcohol toinactivate the polymerization catalyst and the polymerization reactionwas stopped.

Next, 100 parts of the polymerization reaction solution containing anobtained ring-opened polymer was added with 270 parts of cyclohexane,furthermore, added with 5 parts of nickel-alumina catalyst (made byNikki Chemical Co., Ltd.) as a hydrogenated catalyst, applied pressureof 5 MPa by hydrogen, and heated to a temperature of 200° C. whileagitating to bring reaction for 4 hours. Consequently, a reactionsolution containing 20% of a DCP/TCD/ETD ring-opened polymerhydrogenated polymer was obtained.

Next, after removing the hydrogenated catalyst from the solution byfiltering, 0.1 part of a soft polymer (SEPTON2002 made by Kuraray Co.,Ltd.) and 0.1 part of an antioxidant (IRGANOX made by Ciba SpecialtyChemicals K. K.) with respect to 100 parts of the polymer were added tothe obtained solution and dissolved.

Next, cyclohexane and other volatile components were removed from thesolution by using a cylinder shaped concentrating and drying device(made by Hitachi Ltd.), a molten hydrogenated polymer was extruded to bea strand shape from an extruding machine, cooled and pelletized toobtain a ring-opened polymer hydrogenate. The hydrogenate had a glasstransition temperature,Tg of 140° C.

The obtained pellets are subjected to melt extrusion by a single-screwextrusion molding machine (made by Japan Steel Works Ltd.) having acylinder inner diameter of 50 mm and a screw L/D value of 28 at a barreltemperature of 260° C., and a sheet molten resin having a width of 650mm was extruded from a coat hunger die at a die temperature of 260° C.The result was made close to a first cooling drum (diameter of 200 mm,temperature T₁ of 135° C., rotation speed R₁ of 14.50 m/min.),immediately transferred by a knife coater from the first cooling drum,then, to a second cooling drum (diameter of 350 mm, temperature T₂ of125° C., rotation speed R₂ of 14.46 m/min.), then, to a third coolingdrum (diameter of 350 mm, temperature T₃ of 80° C., rotation speed R₃ of14.40 m/min.) successively, and cooling and smoothing of the front andback surfaces by transferring the cool drum surfaces were sequentiallyperformed to obtain a thermoplastic resin film having a width of 550 mm(the neck-in was 50 mm on the right and the left). The thermoplasticresin film passed through an adjustment drum, then, 30 mm was removed asselvage from both ends by a cutter, the result was wound in a roll shapeby a wind drum, and a rolled optical film was obtained.

Note that a contact time t₁ of the sheet thermoplastic resin with thefirst cooling drum was 3.1 (seconds), a resin temperature Tp₁ whenmoving away from the first cooling drum was 132 (° C.), a resintemperature Tp₂ when moving away from the second cooling drum was 119 (°C.), and a resin temperature Tp₃ when moving away from the third coolingdrum was 79 (° C.). (R₃/R₂)=0.996, (R₂/R₁)=0.997 and (t₁×(Tp₁−Tg))=−25(unit: sec.·deg).

The above test items were conducted on the obtained optical film, andthe results are shown in Table 1.

Example 2

Except for changing T₁ in the example 1 to 130° C., an optical film wasproduced in the same way as in the example 1. At this time, a contacttime t₁ of a sheet thermoplastic resin with the first cooling drum was3.1 (sec.), a resin temperature Tp₁ when moving away from the firstcooling drum was 128 (° C.), a resin temperature Tp₂ when moving awayfrom the second cooling drum was 121 (° C.), a resin temperature Tp₃when moving away from the third cooling drum was 81 (° C.), andt₁×(Tp₁−Tg) was −37 (unit: sec.·deg). Test results of an obtainedoptical film are shown in Table 1.

Example 3

Except for changing T₁ in the example 1 to 130° C. and T₂ to 120° C., anoptical film was produced in the same way as in the example 1. At thistime, a contact time t₁ of a sheet thermoplastic resin with the firstcooling drum was 3.1 (sec.), a resin temperature Tp₁ when moving awayfrom the first cooling drum was 128 (° C.), a resin temperature Tp₂ whenmoving away from the second cooling drum was 114 (° C.) , a resintemperature Tp₃ when moving away from the third cooling drum was 77 (°C.), and t₁×(Tp₁−Tg) was −37 (unit: sec.·deg). Test results of anobtained optical film are shown in Table 1.

Example 4

Except for changing T₁ in the example 1 to 140° C. and T₂ to 100° C., anoptical film was produced in the same way as in the example 1. At thistime, a contact time t₁ of a sheet thermoplastic resin with the firstcooling drum was 3.1 (sec.), a resin temperature Tp₁ when moving awayfrom the first cooling drum was 137 (° C.), a resin temperature Tp₂ whenmoving away from the second cooling drum was 96 (° C.), a resintemperature Tp₃ when moving away from the third cooling drum was 75 (°C.), and t₁×(Tp₁−Tg) was −9 (unit: sec.·deg). Test results of anobtained optical film are shown in Table 1.

Comparative Example 1

Except for changing T₁ in the example 1 to 125° C., an optical film wasproduced in the same way as in the example 1. At this time, a contacttime t₁ of a sheet thermoplastic resin with the first cooling drum was3.1 (sec.), a resin temperature Tp₁ when moving away from the firstcooling drum was 123 (° C.), a resin temperature Tp₂ when moving awayfrom the second cooling drum was 117 (° C.), a resin temperature Tp₃when moving away from the third cooling drum was 79 (° C.), andt₁×(Tp₁−Tg) was −53 (unit: sec.·deg). Test results of an obtainedoptical film are shown in Table 1.

Comparative Example 2

Except for changing T₁ in the example 1 to 125° C. and T₂ to 120° C., anoptical film was produced in the same way as in the example 1. At thistime, a contact time t₁ of a sheet thermoplastic resin with the firstcooling drum was 3.1 (sec.), a resin temperature Tp₁ when moving awayfrom the first cooling drum was 123 (° C.), a resin temperature Tp₂ whenmoving away from the second cooling drum was 115 (° C.), a resintemperature Tp₃ when moving away from the third cooling drum was 76 (°C.), and t₁×(Tp₁−Tg) was −53 (unit: sec.·deg). Test results of anobtained optical film are shown in Table 1.

Example 5

Except for changing the width of the selvage to 150 mm on the right andon the left, an optical film was produced in the same way as in thecomparative example 2. Test results of an obtained optical film areshown in Table 1. This example shows a result of testing by obtaining anoptical film satisfying the requirements of the present invention bychanging a width of the selvage comparing with the case of thecomparative example 2.

Note that various conditions at the time of producing a film are shownin Table 2. TABLE 1 Result of Test on Characteristic Value andPerformance Comparative Comparative Example Example Example ExampleExample Example Example 1 2 3 4 5 1 2 Characteristic Value Thickness(μm) 52 47 47 49 44 48 44 Maximum Re (nm) 2 2 8 15 4 2 8 Range of α(rad) 0.014˜ 0.011˜ 0.008˜ 0.007˜ 0.007˜ 0.035˜2.81  0.040˜3.26  2.772.89 3.11 3.09 3.05 Range of sin² 2α 0.008˜ 0.004˜ 0.003˜ 0.002˜ 0.002˜0.005˜0.997 0.007˜0.998 0.998 0.999 0.998 1.000 0.998 Range of sin²(π ·Re/550) × 10⁻⁵ 0˜1.3 0˜13.0 0˜208 13˜731 0˜52  0˜13  0˜208 Maximum Zvalue × 10⁻⁵ 0.8 3.3 2.3 3.6 3.5 4.5 6.4 Performance Average Whole LightTransmittance 0.02 0.02 0.02 0.02 0.02 0.04 0.04 (%) Color Unevenness ⊚◯ ◯ ◯ ◯ Δ X Simple Liquid Crystal Display Test ◯ ◯ ◯ ◯ ◯ Δ X ColorUnevenness after Durability Test ⊚ ◯ ◯ Δ Δ X X

TABLE 2 Conditions Differ- T1 T2 ence of T3 Tp1 Tp2 Tp3 R1 R2 R3 Tg t1 ×(° C.) (° C.) T1 and T2 (° C.) (° C.) (° C.) (° C.) t1(s) (m/min.)(m/min.) (m/min.) R3/R2 R2/R1 (° C.) (Tp1 − Tg) Example 1 135 125 10 80132 119 79 3.1 14.50 14.46 14.40 0.996 0.997 140 −25 Example 2 130 125 580 128 121 81 3.1 14.50 14.46 14.40 0.996 0.997 140 −37 Example 3 130120 10 80 128 114 77 3.1 14.50 14.46 14.40 0.996 0.997 140 −37 Example 4140 100 40 80 137 96 75 3.1 14.50 14.46 14.40 0.996 0.997 140 −9Compara- 125 125 0 80 123 117 79 3.1 14.50 14.46 14.40 0.996 0.997 140−53 tive Example 1 Compara- 125 120 5 80 123 115 76 3.1 14.50 14.4614.40 0.996 0.997 140 −53 tive Example 2 Example 5 125 120 5 80 123 11576 3.1 14.50 14.46 14.40 0.996 0.997 140 −53

As shown in Table 1, the optical films of examples of the presentinvention have a small Z value comparing with those in the comparativeexamples, so that they are excellent in the whole light transmittance,polarization degree, color unevenness and color unevenness after adurability test. In the results of the examples 1 to 3 and 5, themaximum Re values are smaller than the result of the example 4, so thatcolor unevenness and results of the simple liquid crystal display testare excellent. In the results of the examples 1 to 3, the Z values aresmaller than those in the results of the examples 4 and 5, so that colorunevenness after a durability test is excellent.

Next, a performance test in the case of performing stretch processing onthe obtained optical film to obtain a phase difference film wasconducted. The optical film obtained in the present invention can bemade to be a phase difference film by performing stretch processingthereafter. A phase difference film is required to have a predeterminedphase difference (retardation value), and the value has to be uniform.To examine performance of the optical film of the present invention as aphase difference film as a raw material film, a test below wasconducted.

An obtained optical film obtained in the experiment was cut out to be atest piece (a width of 100 mm and a length of 150 mm), and the testpiece was stretched at 140° C. by 1.1 to 2 times in the longitudinaldirection at a speed of 100 mm/min. The stretching ratio was adjusted sothat an average retardation becomes 275±10 nm. The retardation wasmeasured by the same measurement method as that in the (2), and Reunevenness was obtained by dividing unevenness (difference between themaximum value and the minimum value) by an average value. Themeasurement points were at a center portion in the width direction and10 points at every 10 mm near the center in the length direction.

Table 3 shows results of measuring Re unevenness after stretching inrespective examples. TABLE 3 Result of Re Unevenness Test afterStretching Comparative Comparative Example 1 Example 2 Example 3 Example4 Example 5 Example 1 Example 2 Re Unevenness Test 1.8 2.2 2.1 2.6 2.15.8 6.6 after Stretching (%)

As shown in Table 3, the optical films of the examples of the presentinvention have smaller Z values than those in the case of comparativeexamples, so that retardation unevenness after stretch processing issmall, which is excellent. Thus, the optical film of the presentinvention is suitable as a raw material film of a phase difference film.

Industrial Applicability

An optical film with small optical distortion is provided by the presentinvention. The optical film of the present invention is useful as avariety of optical films of a flat panel display, such as a protectivefilm of a polarizer and a liquid crystal cell substrate film. Also, theoptical film of the present invention has small and uniform retardation,and it can be made to be a phase difference film having a predetermineduniform retardation by performing stretch processing after that.Therefore, the optical film of the present invention is useful as a rawmaterial film of a phase difference film, etc.

1. An optical film composed of a thermoplastic resin film obtained byusing a melt extruding machine, characterized in that said thermoplasticresin film satisfies a relation of the formula below over the wholesurface of the film when an angle made by the extruding direction of thethermoplastic resin film from the melt extruding machine and a slowphase axis at each point is α, and a retardation amount at each point isRe.[sin² 2α]×[sin²(π·Re/550)]≦4.0×10⁻⁵
 2. The optical film as set forth inclaim 1, wherein a value of said Re is 10 nm or less.
 3. The opticalfilm as set forth in claim 1 or 2, wherein said thermoplastic resin isan alicyclic structure containing polymer.
 4. A production method of anoptical film composed of a thermoplastic resin film, including a step ofcooling a molten thermoplastic resin extruded from an extruding machineby making it successively circumscribed with a first cooling drum, asecond cooling drum and a third cooling drum: wherein, when assumingthat rotation speed of said third cooling drum is R₃ (m/min.), androtation speed of said second cooling drum is R₂ (m/min.), a ratio ofthe R₃ and R₂ (R₃/R₂) is made to be 0.990 or more but less than 0.999 tocool said thermoplastic resin.
 5. The production method of an opticalfilm as set forth in claim 4, wherein, when assuming a resin contacttime in said first cooling drum is t₁ (sec.), a temperature when saidthermoplastic resin moves away from said first cooling drum is Tp₁ (°C.), and a glass transition temperature of said thermoplastic resin isTg (° C.), t₁×(Tp₁−Tg) (unit: sec.·deg) is made to be −50 or higher and20 or lower to cool said thermoplastic resin.
 6. The production methodof an optical film as set forth in claim 4, wherein, when assuming thatrotation speed of said first cooling drum is R₁ (m/min.), a ratio of theR₂ and R₁ (R₂/R₁ ) is made to be 0.990 or more but less than 1.01 tocool said thermoplastic resin.
 7. The production method of an opticalfilm as set forth in claim 4, wherein, when assuming that a temperaturewhen said thermoplastic resin moves away from said third cooling drum isTp₃ (° C.), the Tp₃ is made to be a lower temperature than said Tg by 50to 100° C. to cool said thermoplastic resin.
 8. The production method ofan optical film as set forth in claim 4, wherein, when assuming that atemperature when said thermoplastic resin moves away from said secondcooling drum is Tp₂ (° C.), the Tp₂ is made to be a lower temperaturethan said Tg by 0 to 60° C. to cool said thermoplastic resin.
 9. Theproduction method of an optical film as set forth in claim 4, wherein, atemperature difference of said first cooling drum and said secondcooling drum is made to be 20° C. or less to cool said thermoplasticresin.
 10. A protective film of a polarizer composed of the optical filmas set forth in claim
 1. 11. A polarizing film having a polarizer and aprotective film as set froth in claim 10 stacked on one surface or bothsurfaces of the polarizer via an adhesive layer.
 12. A phase differencefilm obtained by performing stretch processing on the optical film asset froth in claim 1.